C# – If a struct cannot inherit another class or struct, why does Int32 have a ToString() method?

cinheritancenetstruct

int a = 2;

Console.WriteLine(a.ToString()); // displays 2

// definition of ToString() here - public override string ToString();

Now, here are some of my understandings:

All the classes in .net get a ToString() method, which is inherited from the Object class.
A structure cannot be derived from a Class or another struct. int is a structure of type Int32, which gets a couple of ToString() [With Parameters] methods from the Interfaces which it implements.
There is also a ToString() [without params] function in struct Int32

According to http://msdn.microsoft.com/en-us/library/system.int32.tostring.aspx,

struct Int32 overrides ValueType.ToString() method

If a struct cannot inherit some class or struct, can you please explain how this ToString() method is available for Int32?

Best Answer

If a struct cannot inherit some class or struct,

This isn't true. All structs (and the built-in value types, like System.Int32, System.Single, etc) always implicitly inherit from System.ValueType (which, in turn, inherits from System.Object).

However, you can't make a struct that inherits from anything else.

This is clearly spelled out in the C# language spec, 4.1.1:

4.1.1 The System.ValueType type

All value types implicitly inherit from the class System.ValueType, which, in turn, inherits from class object. It is not possible for any type to derive from a value type, and value types are thus implicitly sealed (§10.1.1.2).

Then, later (4.1.3) struct is explicitly defined to be a value type:

4.1.3 Struct types

A struct type is a value type that can declare constants, fields, methods, properties, indexers, operators, instance constructors, static constructors, and nested types.

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C# – Multiple Inheritance in C#

Consider just using composition instead of trying to simulate Multiple Inheritance. You can use Interfaces to define what classes make up the composition, eg: ISteerable implies a property of type SteeringWheel, IBrakable implies a property of type BrakePedal, etc.

Once you've done that, you could use the Extension Methods feature added to C# 3.0 to further simplify calling methods on those implied properties, eg:

public interface ISteerable { SteeringWheel wheel { get; set; } }

public interface IBrakable { BrakePedal brake { get; set; } }

public class Vehicle : ISteerable, IBrakable
{
    public SteeringWheel wheel { get; set; }

    public BrakePedal brake { get; set; }

    public Vehicle() { wheel = new SteeringWheel(); brake = new BrakePedal(); }
}

public static class SteeringExtensions
{
    public static void SteerLeft(this ISteerable vehicle)
    {
        vehicle.wheel.SteerLeft();
    }
}

public static class BrakeExtensions
{
    public static void Stop(this IBrakable vehicle)
    {
        vehicle.brake.ApplyUntilStop();
    }
}


public class Main
{
    Vehicle myCar = new Vehicle();

    public void main()
    {
        myCar.SteerLeft();
        myCar.Stop();
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}

C# – Why does implicitly calling toString on a value type cause a box instruction

You are not implicitly calling ToString at all. The is no overload of the WriteLine method that takes strings after the format string, it only takes objects.

So, you are not implicitly calling ToString, you are implicitly converting the int to object. The first case is equivalent to:

Console.WriteLine("val is {0}", (object)val);

As the int is a value type, boxing occurs.

The second case is equivalent to:

Console.WriteLine("val is {0}", (object)val.ToString());

As the string is a reference type, casting it to object doesn't actually cause any code to be emitted. It just matches the type with the method signature.

Best Answer

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C# – Multiple Inheritance in C#

C# – Why does implicitly calling toString on a value type cause a box instruction

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